(1) Field of the Invention
The present invention relates to a coating composition for metal coating, and more particularly to a coating composition for metal coating, which is used in a so-called precoated metal sheet, in which a post processing is applied to a dry film formed by coating, curing and drying.
(2) Description of the Prior Art
The following various resin-based coating compositions are commercially available as the coating composition for metal coating. Examples of the above coating compositions may include lacquer type coating compositions such as vinyl chloride resin based coating composition, cellulose acetate resin based coating composition and the like, and heat curing type coating compositions such as epoxy-amino resin based coating composition, epoxy-phenol resin based coating composition, polyester-amino resin based coating composition, acryl-amino resin based coating composition and the like.
In recent years, needs of short time curing are increasing from the standpoints of economizing resources and energy.
However, the coating composition in the prior art, for example, the lacquer type coating composition is applicable to short time curing, but raises problems in processability and corrosion resistance.
The above heat curing type coating composition, for example, is incapable of obtaining a sufficiently cured film by short time curing, and consequently raises problems in processability, corrosion resistance, etc. Increase in the curing temperature to obtain a sufficiently cured film causes a violent reaction to take place and causes a remarkable foaming to take place in the film without solving the above problems. On the other hand, Japanese Patent Application Laid-Open No. 268768/86, for example, discloses a coating composition having suitability for the short time curing. The use of the above coating composition solves the problem of the foaming developed in the film under the curing conditions of high temperatures and short period of time. However, the above coating composition is not satisfactory in the processability and corrosion resistance under severe conditions because of film brittleness considered to be caused by self-condensation of a resol phenol resin conventionally used as a curing agent.